Joining the U.S. Army as a Prior Service Marine. What advice do you guys have for someone making the switch?

Read the regulations instead of taking everyone's word for things. Become familiar with AR and DA Pam 670-1 and AR 600-8-22 when it comes to your USMC awards and decorations. That will be one of your future questions. These are also good to know:

https://www.hrc.army.mil/content/Sister%20Service%20Awards

Also familiarize yourself with AR 600–8–104 to update your records. There is nothing like HR soldiers who are misinformed.

Don't be that guy simple as that....

I see it all the time from marines who switch over to the army all they ever talk about is marine corps this marine corps that. Don't be that guy. There are a lot of similarities and differences between the branches. Learn to like and dislike them all. there are tons of things I loved and hated about the corps and tons of things I love and hate about the Army. Just learn the basic customs and courtesies and become the best Nco you can learn from everyone around you. There will be a learning curve with a lot of things.

To this day I cannot stand a beret and never will.